Due to the tumbling action and gas flow patterns, rotary kilns generate dust. This dust consists of the fines in the feed materials and fines generated by the breakdown of larger feed particles due to attrition. To date, no one has been able to eliminate dust generation in rotary kilns.
This invention is a process by which two opposing effects are used to maintain a desired flame geometry in a rotary kiln. Dust insufflation will cool and lengthen the flame in a rotary kiln. Oxygen addition will shorten and intensify it. By suitably proportioning dust and oxygen addition while properly fueling the furnace, the flame geometry required for a particular rotary kiln is maintained while dust utilization is increased.
During the thermal processing of mineral products a certain amount of dust is entrained in the gas system exhausting the kiln. This dust is primarily composed of partially processed product. Some of the dust may be completely processed product, unburned carbon, condensates and eroded furnace lining. The dust is usually collected in an environmental control system (baghouse, cyclone separator, electrostatic precipitator, etc.) to keep the furnace particulate emissions within the air quality guidelines.
This dust is not marketable as the originally intended finished product. It presents a disposal problem and is sometimes hazardous. The amount of dust generated can vary widely but is typically 4 to 15% of the theoretical yield of product.
If this dust can be recycled into the product, a disposal cost is eliminated and production can be increased with no cost increases upstream of the kiln (i.e. mining, grinding, etc.)
Heretofore little or no waste dust could be recycled into the kiln. Mixing with kiln feed does not work because the fine dust particles become entrained in the counter flow (flue) gas stream. Introduction into the furnace hot end produces a lengthening of the flame and a cooling in the flame temperature causing lower heat flux and incomplete heat treatment of the product.
Some dust has been successfully recycled in wet process cement kilns. This technique. known as insufflation, is very limited, however, in the amount of dust which can be recycled. Insufflation has been done through the fuel burner pipe and also through dust injection pipes located near the burner pipe. The most common position of the dust injection pipe is above and parallel to the burner pipe. slightly offset from directly above the burner pipe.
Previous recycle attempts have had limited success for a number of reasons. The primary reason is that the dust decreases the rate of the combustion reaction and thereby lowers the flame temperature. Other undesirable operational effects include high CO emissions, increase in the cold end kiln temperature, too long a flame, product increases greater amounts of incomplete clinker formation, low free lime, and increased cold end dust generation.
Historically, high dust losses were not a high priority concern until government land reclamation laws such as the Resource Conservation and Recovery Act (RCRA) affected disposal. Costs associated with mining and feed preparation are not a significant part of production cost, as are product firing cost.